20 Apr 9 Nuclear, Strong Force, Fission With Brainstorming

Today: More about half-life, why atoms are radioactive, fission Diagram of neutron-induced U-235 fission

Half-life is the average amount of time for ½ of the atoms to decay 100% remaining 50% remaining 25% remaining 12.5% remaining Time Isotope with long half-life Isotope with short half-life “ Exponential Decay”

Radium-226 Where does it come from? ,[object Object],[object Object],[object Object],[object Object]

Radium-226 Where does it come from? ,[object Object],It turns out it comes from the radioactive decay of uranium in natural deposits. Where does the Uranium come from??? http://www.youtube.com/watch?v=6SxzfZ8bRO4&feature=related

Why are some atoms radioactive? First: Some brainstorming Form small groups, with a spokesperson How many different forces in nature can you think of? Can you classify them into similar groups?

Brainstorming results ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

First, we need to wonder why a nucleus can exist at all! ,[object Object],[object Object],[object Object],Same charges repel each other http://www.youtube.com/watch?v=NiwBwSAeDu0 Force  Charge1 * Charge2 distance 2

The strong force holds quarks together ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

Only gravity and electrical force act at long distances ,[object Object],[object Object],[object Object],[object Object]

Clicker question—fundamental forces ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

Why are some atoms radioactive? ,[object Object],[object Object],[object Object],chemeducator.org

Why are some atoms radioactive? ,[object Object],[object Object],[object Object],U 238 92 Th 234 90 He 4 2 The alpha particle is moving at 15,000 km/s, Having a LOT of kinetic energy Alpha-decay

Why are some atoms radioactive? ,[object Object],U 238 92 Th 234 90 He 4 2 The alpha particle is moving at 15,000 km/s, Having a LOT of kinetic energy Alpha-decay (heat up reaction) Height of “activation barrier” determines reaction rate

Why are some atoms radioactive? The nuclear (residual strong) force presents an activation barrier Quantum mechanics allows some probability that an alpha particle can “tunnel” through the barrier Once through, electrostatic force dominates (imagine ripping apart velcro)

Take home message for “why radioactivity” ,[object Object],[object Object],[object Object],[object Object]

Alpha decay can be though of as a very common but special form of nuclear fission . ,[object Object],[object Object],[object Object]

As nuclei get larger, electrostatic force “wins” Strong force large between close protons. Strong force much lower between distal protons But electrostatic force still very high! (Nucleons on the surface feel less strong attraction.)

You can imagine large nuclei like large soap bubbles…very floppy Small Medium Large

If the nucleus randomly deforms, it can become favorable to split into two! ,[object Object],& free neutrons!!! http://www.youtube.com/watch?v=jk6Hm1QoDYY

However, free neutrons can be absorbed and induce fission ,[object Object]

Naturally occurring radium is a mixture of mostly U-238 and a small amount of U-235. ,[object Object],[object Object],[object Object],[object Object],[object Object]

Because of mix of mostly U-238, natural uranium is not suitable for a chain reaction. ,[object Object],[object Object],[object Object]

For U-238 and U-235, probability of absorbing neutron depends on neutron energy ,[object Object],[object Object],[object Object],[object Object]

Clicker question – Nuclear Reactor Physics ,[object Object],[object Object],[object Object],[object Object],[object Object]

Clicker question – Nuclear Reactor Physics ,[object Object],[object Object],[object Object],[object Object],[object Object],This was the key to creating the first man-made controlled nuclear chain reaction…neutrons were slowed down by graphite

Clicker question—Ratio of U-238 to U-235 ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

Clicker question—Ratio of U-238 to U-235 ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],Proportion was high enough To help with random creation of Nuclear reactors! (About 3% U-235)

20 Apr 9 Nuclear, Strong Force, Fission With Brainstorming

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20 Apr 9 Nuclear, Strong Force, Fission With Brainstorming